Lamps such as those described above usually have a light source capsule that is enclosed in an outer envelope that can be evacuated or contain an inert gas. The light source capsule can be subject to bursting if its internal pressure is greater than or less than the pressure of the gas surrounding the capsule. A burst of a light source capsule can shatter the outer envelope and thereby create a dangerous situation. To provide a measure of protection from such bursts it has been the industry practice to enclose the lamp in a protective fixture or to provide an unusually robust outer envelope to contain any shards from the burst capsule.
In particular, metal halide arc discharge lamps are frequently employed in commercial usage because of their high luminous efficacy and long life. A typical metal halide arc discharge lamp includes a quartz or fused silica arc tube that is hermetically sealed within a borosilicate glass outer envelope. The arc tube, itself hermetically sealed, has tungsten electrodes sealed into opposite ends and contains a fill material that can include mercury, as well as metal halide additives, and a rare gas to facilitate starting. In some cases, particularly in high wattage lamps, the outer envelope is filled with nitrogen or another inert gas at less than atmospheric pressure. In other cases, particularly in low wattage lamps, the outer envelope is evacuated.
It has been found desirable to provide such lamps, and in particular, metal halide arc discharge lamps with a shroud that comprises a generally light-transmissive member, such as quartz, that is able to withstand high operating temperatures. The arc tube and the shroud are coaxially mounted within the lamp envelope with the arc tube located within the shroud. Preferably, the shroud is tubular and open at both ends. In other cases, the shroud is open on one end and has a domed configuration on the other end. Shrouds for metal halide arc discharge lamps are disclosed in U.S. Pat. No. 4,499,396 issued Feb. 12, 1985 to Fohl et al. and U.S. Pat. No. 4,580,989 issued Apr. 8, 1986 to Fohl et al. See also U.S. Pat. No. 4,281,274 issued Jul. 28, 1981 to Bechard et al.
The shroud has several beneficial effects on lamp operation. In lamps with a gas-filled outer envelope, the shroud reduces convective heat losses from the arc tube and thereby improves the luminous output and the color temperature of the lamp. In lamps with an evacuated outer envelope, the shroud helps to elevate and/or equalize the surface temperature of the arc tube. In addition, the shroud effectively reduces sodium losses and improves the maintenance of phosphor efficiency in metal halide lamps having a phosphor coating on the inside surface of the outer envelope. Finally, the shroud improves the safety of the lamp by acting as a containment device in the event that the arc tube shatters.
While these shrouded lamps have received great acceptance in the marketplace, (since lamps so equipped do not require an extensive, enclosed fixture) the use of the quartz shroud adds considerable expense, and considerable weight, to the lamp. Additionally, these lamps employ a wire frame to mount the arc tube and the shroud, and this wire frame can contribute to a loss of sodium from the arc tube, which loss affects the color output of the lamp as well as the life of the lamp and, additionally, contributes an undesired shadow.
Further, the quartz shroud is a single piece that favors a single (or very limited number) continuous ‘global’ fracture when struck by an arc tube shard because of its nearly uniform rigid continuum structure and the fact that crack propagation velocity in quartz tubing is in the neighborhood of ˜2000 m/sec. This velocity is much greater than the nominal shard/envelope impact velocity of about 25 m/sec. Therefore, an initiating crack spreads elsewhere around the shroud before other shards have a chance for their own impacts. This behavior can weaken the tubular shroud at locations other than the initial impact site and can yield relatively large fragmented pieces of shroud and/or light source capsule. Subsequent shard impacts at these other locations are met with significantly reduced barrier strength. The shards are propelled toward the inner surface of the outer envelope by expanding gases from the light source capsule burst. Therefore, it is possible under some conditions for the shroud to contribute to the fracture of the outer envelope, the very situation it was supposed to prevent.